Banking giant HSBC could move around 1,000 jobs from London to Paris if Britain votes to leave the EU, according to its chief executive.
Stuart Gulliver said a Brexit could force the bank to transfer a large tranche of jobs from its UK investment banking arm to its business in France - although the move would depend on the terms of a British exit.
The comments came on the same day HSBC pledged to keep its headquarters in London following a high-profile lengthy review.
Mr Gulliver said the forthcoming referendum had not impacted the bank's decision to keep its headquarters in the British capital.
But he stated that "if the UK leaves the EU it could have a significant impact on our non-ring-fenced bank - our trading room, corporate banking and investment banking - although it would not have an impact on our holding company domicile".
He told Sky News: "We have 5,000 people in global banking and markets in London and I could imagine that around 20% of those would move to Paris."
The decision to move jobs would hinge on the terms of a British exit from the EU, Mr Gulliver said, including whether the UK could still access the financial services "passporting" regime allowing member states to trade across national borders.
Responding to the comments, the Prime Minister's official spokeswoman said: "The PM has talked many times about the issues and choices the British people will have to weigh up when they make a decision on whether the UK should remain or leave, that there are economic risks.
"But he has also talked about the confidence we should have in our country and our ability to succeed."
HSBC revealed last year it was considering whether to move its headquarters elsewhere over concerns about stricter regulations.
HSBC Group chairman Douglas Flint said the decision to keep the London HQ "offered the best outcome for our customers and shareholders".
The announcement comes after Chancellor George Osborne made a series of concessions to the City in recent months.
Mr Gulliver said: "Having our headquarters in the UK and our significant business in Asia Pacific delivers the best of both worlds to our stakeholders."
A Treasury spokeswoman said: "We welcome HSBC's decision. They've looked carefully and dispassionately at the facts and confirmed that the UK is the best place to base a global business.
"It's a vote of confidence in the Government's economic plan, and a boost to our goal of making the UK a great place to do more business with China and the rest of Asia."
The bank's decision was seized upon by campaigners for UK withdrawal from the EU, who said it undermined pro-European claims that Brexit might trigger an exodus of major firms for the continent.
Matthew Elliott, chief executive of the Vote Leave campaign, said: "HSBC claimed that it might move its headquarters due to uncertainty about whether the UK would stay in the EU. This was an attempt to scare people into backing EU membership. Today's announcement shows that even the proponents of Project Fear don't actually believe it."
But Mr Flint reiterated the bank's intentions to move staff to Paris in the event of a British withdrawal.
He told the BBC Radio 4 Today programme: "We are lucky in the sense that we have a major bank in France. So, if we were to leave and if there were to be restrictions, ultimately, on the renegotiation of Britain's position, we have the ability to move activity and people between London and Paris.
"But our strong economic view is that Britain is better within a reformed Europe."
HBSC said the board's decision to keep its headquarters in London was "unanimous".
In its later stages, the review was "narrowed" to the UK and Hong Kong, "both of which are considered by the board to be world-class financial centres", it added.
HSBC's review was announced in April last year at the bank's annual meeting.
It cited increasingly onerous British regulatory conditions and the UK bank levy introduced in 2010 - a tax based on the size of any British-based banks' global balance sheet.